The National Transportation Safety Board on Friday released a metallurgical analysis of approximately 55 feet of pipe that appeared to rule out several possible causes of the pipeline failure, including corrosion or external blunt force, such as from a backhoe, and to suggest the blame could involve substandard welds.

A former real estate agent out of work since May, the 43-year-old mother of five is learning to scale poles and operate a crane, a backhoe and other equipment used at electric and gas company construction sites.